1. Field of the Invention
The present invention relates to an indium oxide powder or indium oxide/tin oxide (hereinafter referred to as "ITO") powder, a sintered body, a process for the preparation thereof, and a use thereof. The ITO sintered body of the present invention is useful as a sputtering target for the formation of a transparent electroconductive film by the sputtering method.
2. Description of Related Art
The demand for an ITO film as a transparent electroconductive film for use in a transparent electrode or touch panel of a solar cell or liquid crystal display has recently been increasing. A method comprising coating finely divided ITO particles on a substrate, a method comprising coating an ITO precursor on a substrate and thermally decomposing the precursor, and a method comprising forming an ITO film on the surface of a substrate by sputtering using an ITO alloy target or ITO sintered body target are known as methods for forming such an ITO film, but at the present, the sputtering method using an ITO sintered body target is the most popular.
The ITO sintered body has heretofore been prepared by compression-molding a mixture of an indium oxide powder and a tin oxide powder and sintering the body. Nevertheless, since the indium oxide powder and tin oxide powder are inherently difficult to sinter, it is not easy to prepare an ITO sintered body having a high density by sintering these powders. The sintered density of the ITO sintered body differs to some extent according to the tin content, and the sintered density corresponding to 100% of the theoretical density is about 7.1 g/cm.sup.3.
In most conventional ITO sintered bodies, many voids are left in the sintered structure and the sintered density is about 60% of the theoretical density the highest, i.e., up to about 4.3 g/cm.sup.3, the specific resistance is larger than 2.times.10.sup.-3 .OMEGA.-cm, and the hue is yellowish green. Since conventional sintered bodies have a low electroconductivity, a low thermal conductivity, and a low mechanical strength, the applicable electric power at the step of forming a film by sputtering is low and therefore, the film-forming rate is low and the discharge state becomes unstable. If excessive electric power is applied so as to increase the film-forming rate, cracking occurs in the sintered body.
Furthermore, when sputtering is carried out by using such a sintered body having a low density, a reduced substance (black substance) is formed on the surface of the sintered body, and if this substance is mingled in a transparent electroconductive film formed on the surface of the substrate, the quality of the film is degraded. Accordingly, every time the reduced substance is formed on the surface of the sintered body, the sputtering operation must be stopped to remove the reduced substance, and this impedes the continuous operation of sputtering.
A conventional sintered body formed by heat pressing a product prepared by a hot press has a relatively high density, but the sintered grain diameter of this ITO sintered body is smaller than 5 .mu.m and it sometimes happens that the sintered body is partially in the reduced state.
If the sintered grain diameter of the sintered body is smaller than 5 .mu.m, the flexural strength of the sintered body is low and the mechanical strength is naturally reduced. Moreover, the presence of the reduced substance deteriorates on the quality of the obtained film, as mentioned above.
With a view to solving these problem, investigations have been made to increase the density and reduce the specific resistance in ITO sintered bodies.
For example, there has been proposed a method in which an indium oxide/tin oxide powder having a relatively large particle diameter, that is, an average particle diameter of 3 to 6 .mu.m, which has been once calcined at a high temperature, is used as the starting material (Japanese Unexamined Patent Publication No. 62-21751). Nevertheless, as seen from the description of examples in this patent publication, the density of the ITO sintered body obtained by using starting material having a relatively large particle diameter is 5 g/cm.sup.3 at highest. There has also been proposed a method in which a co-precipitated ITO powder obtained from a solution containing indium and tin by using a precipitating agent is used as the starting material for obtaining a sintered body (Japanese Unexamined Patent Publication No. 62-12009). Nevertheless, the sintered density of the sintered body obtained according to this method is still about 5 g/cm.sup.3, i.e., about 70% of the theoretical density.
Furthermore, there has been proposed a method in which oxygen deficiency is introduced into an ITO sintered body to reduce the specific resistance (Japanese Unexamined Patent Publication No. 63-40756). This method is effective for reducing the specific resistance of the sintered body, but it is difficult to obtain a sintered body having a high density.
A method for forming an ITO sintered body having a high density by compression-sintering (hot-pressing) an ITO powder has been proposed. The apparatus used for carrying out this method is expensive and the operation is complicated. Moreover, although it is said that a sintered body having a relatively high density can be obtained according to this method, the method is industrially not advantageous.
As the method for preparing the oxide powder, that is, the indium oxide powder, tin oxide powder or ITO powder, there are known a method in which a powder of a corresponding metal hydroxide, oxide hydrate, organic metal salt or inorganic metal salt, or a sol or gel thereof is heat-dehydrated or thermally decomposed, and a method in which a precipitation product formed by adding a precipitating agent to an aqueous solution of a mixture of an indium salt and a tin salt or a hydrolysis product formed from this aqueous solution is thermally decomposed (Japanese Unexamined Patent Publication No. 62-7627, Japanese Unexamined Patent Publication No. 60-186416 and Japanese Unexamined Patent Publication No. 58-36925). An ITO powder having a high purity can be obtained by the method previously proposed by the present inventors, in which a mixed organic acid salt prepared from an aqueous solution of mixed organic acids containing indium and tin is thermally decomposed (Japanese Unexamined Patent Publication No. 63-195101).
As is apparent from the foregoing description, an ITO sintered body having a high density and a low specific resistance has not been prepared according to an industrial method.